Warning! Insert under construction digging man here...

Here are some of the online resources that seem to be excellent for introductory Python. My main criteria are: free, don't require registration, provide tutorials if possible not just reference material.

Python Room

https://pythonroom.com - I have only had a quick look at this site so far but it looks excellent! For instance, I fumbled the closing parenthesis on one of the mini-challenges it gives you as you learn the basics, and it told me what I did wrong - it didn't just give me a syntax error.

Hour of Python

https://hourofpython.com - this site is hosted by trinket.io and that happens to be a commonly recommended Python online development environment. I haven't had a chance to try it out to see if it is really just an hour.